Medical lead having a bandstop filter employing a capacitor and an inductor tank circuit to enhance mri compatibility

ABSTRACT

A bandstop filter includes a capacitance in parallel with an inductance and is placed in series with the implantable lead of an active implantable medical device, wherein values of capacitance and inductance are selected such that the bandstop filter attenuates RF current flow at a selected center MRI RF pulsed frequency or across a range of frequencies. The Q i  of the inductor and the Q c  of the capacitor are controlled to reduce the overall Q of the bandstop filter to attenuate current flow through the implantable lead along a range of selected frequencies. In a preferred form, the bandstop filter is integrated into a Tip and/or Ring electrode for the active implantable medical device.

BACKGROUND OF THE INVENTION

This invention relates generally to medical leads of the type used withactive implantable medical devices (AIMDs) such as cardiac pacemakers,cardioverter defibrillators, neurostimulators, and the like, whichemploy a bandstop filter to decouple the implantable leads and/orelectronic components of the implantable medical device from undesirableelectromagnetic interference (EMI) signals at a selected frequency oracross a range of frequencies, such as the RF pulsed fields of MagneticResonance Imaging (MRI) equipment.

Compatibility of cardiac pacemakers, implantable defibrillators andother types of active implantable medical devices with magneticresonance imaging (MRI) and other types of hospital diagnostic equipmenthas become a major issue. If one goes to the websites of the majorcardiac pacemaker manufacturers in the United States, which include St.Jude Medical, Medtronic and Boston Scientific (formerly Guidant), onewill see that the use of MRI is generally contra-indicated withpacemakers and implantable defibrillators. See also:

(1) “Safety Aspects of Cardiac Pacemakers in Magnetic ResonanceImaging”, a dissertation submitted to the Swiss Federal Institute ofTechnology Zurich presented by Roger Christoph Luchinger, Zurich 2002;

(2) “I. Dielectric Properties of Biological Tissues: Literature Survey”,by C. Gabriel, S. Gabriel and E. Cortout; (3) “II. Dielectric Propertiesof Biological Tissues: Measurements and the Frequency Range 0 Hz to 20GHz”, by S. Gabriel, R. W. Lau and C. Gabriel; (4) “III. DielectricProperties of Biological Tissues: Parametric Models for the DielectricSpectrum of Tissues”, by S. Gabriel, R. W. Lau and C. Gabriel; and (5)“Advanced Engineering Electromagnetics, C. A. Balanis, Wiley, 1989;

(6) Systems and Methods for Magnetic-Resonance-Guided InterventionalProcedures, U.S. Pat. No. 7,844,319;(7) Multifunctional Interventional Devices for MRI: A CombinedElectrophysiology/MRI Catheter, by, Robert C. Susil, Henry R. Halperin,Christopher j. Yeung, Albert C. Lardo and Ergin Atalar, MRI in Medicine,2002; and(8) Multifunctional Interventional Devices for Use in MRI, U.S. PatentApplication Ser. No. 60/283,725, filed Apr. 13, 2001.The contents of the foregoing are all incorporated herein by reference.

However, an extensive review of the literature indicates that MRI isindeed often used with pacemaker, neurostimulator and other activeimplantable medical device (AIMD) patients. The safety and feasibilityof MRI in patients with cardiac pacemakers is an issue of gainingsignificance. The effects of MRI on patients' pacemaker systems haveonly been analyzed retrospectively in some case reports. There are anumber of papers that indicate that MRI on new generation pacemakers canbe conducted up to 0.5 Tesla (T). MRI is one of medicine's most valuablediagnostic tools. MRI is, of course, extensively used for imaging, butis also used for interventional medicine (surgery). In addition, MRI isused in real time to guide ablation catheters, neurostimulator tips,deep brain probes and the like. An absolute contra-indication forpacemaker patients means that pacemaker and ICD wearers are excludedfrom MRI. This is particularly true of scans of the thorax and abdominalareas. Because of MRI's incredible value as a diagnostic tool forimaging organs and other body tissues, many physicians simply take therisk and go ahead and perform MRI on a pacemaker patient. The literatureindicates a number of precautions that physicians should take in thiscase, including limiting the power of the MRI RF Pulsed field (SpecificAbsorption Rate-SAR level), programming the pacemaker to fixed orasynchronous pacing mode, and then careful reprogramming and evaluationof the pacemaker and patient after the procedure is complete. There havebeen reports of latent problems with cardiac pacemakers or other AIMDsafter an MRI procedure sometimes occurring many days later. Moreover,there are a number of recent papers that indicate that the SAR level isnot entirely predictive of the heating that would be found in implantedleads or devices. For example, for magnetic resonance imaging devicesoperating at the same magnetic field strength and also at the same SARlevel, considerable variations have been found relative to heating ofimplanted leads. It is speculated that SAR level alone is not a goodpredictor of whether or not an implanted device or its associated leadsystem will overheat.

There are three types of electromagnetic fields used in an MRI unit. Thefirst type is the main static magnetic field designated B₀ which is usedto align protons in body tissue. The field strength varies from 0.5 to3.0 Tesla in most of the currently available MRI units in clinical use.Some of the newer MRI system fields can go as high as 4 to 5 Tesla. Atthe recent International Society for Magnetic Resonance in Medicine(ISMRM), which was held on 5 and 6 Nov. 2005, it was reported thatcertain research systems are going up as high as 11.7 Tesla. This isover 100,000 times the magnetic field strength of the earth. A staticmagnetic field can induce powerful mechanical forces and torque on anymagnetic materials implanted within the patient. This would includecertain components within the cardiac pacemaker itself and or leadsystems. It is not likely (other than sudden system shut down) that thestatic MRI magnetic field can induce currents into the pacemaker leadsystem and hence into the pacemaker itself. It is a basic principle ofphysics that a magnetic field must either be time-varying as it cutsacross the conductor, or the conductor itself must move within themagnetic field for currents to be induced.

The second type of field produced by magnetic resonance imaging is thepulsed RF field which is generated by the body coil or head coil. Thisis used to change the energy state of the protons and illicit MRIsignals from tissue. The RF field is homogeneous in the central regionand has two main components: (1) the magnetic field is circularlypolarized in the actual plane; and (2) the electric field is related tothe magnetic field by Maxwell's equations. In general, the RF field isswitched on and off during measurements and usually has a frequency of21 MHz to 64 MHz to 128 MHz depending upon the static magnetic fieldstrength. The frequency of the RF pulse varies with the field strengthof the main static field where, for a hydrogen MRI system, RF PULSEDFREQUENCY in MHz=(42.56) (STATIC FIELD STRENGTH IN TESLA).

The third type of electromagnetic field is the time-varying magneticgradient fields designated Bi which are used for spatial localization.These change their strength along different orientations and operatingfrequencies on the order of 1 kHz. The vectors of the magnetic fieldgradients in the X, Y and Z directions are produced by three sets oforthogonally positioned coils and are switched on only during themeasurements. In some cases, the gradient field has been shown toelevate natural heart rhythms (heart beat). This is not completelyunderstood, but it is a repeatable phenomenon. The gradient field is notconsidered by many researchers to create any other adverse effects.

It is instructive to note how voltages and EMI are induced into animplanted lead system. At very low frequency (VLF), voltages are inducedat the input to the cardiac pacemaker as currents circulate throughoutthe patient's body and create voltage drops. Because of the vectordisplacement between the pacemaker housing and, for example, the Tipelectrode, voltage drop across the resistance of body tissues may besensed due to Ohms Law and the circulating current of the RF signal. Athigher frequencies, the implanted lead systems actually act as antennaswhere currents are induced along their length. These antennas are notvery efficient due to the damping effects of body tissue; however, thiscan often be offset by extremely high power fields (such as MRI pulsedfields) and/or body resonances. At very high frequencies (such ascellular telephone frequencies), EMI signals are induced only into thefirst area of the lead system (for example, at the header block of acardiac pacemaker). This has to do with the wavelength of the signalsinvolved and where they couple efficiently into the system.

Magnetic field coupling into an implanted lead system is based on loopareas. For example, in a cardiac pacemaker, there is a loop formed bythe lead as it comes from the cardiac pacemaker housing to its distalTip, for example, located in the right ventricle. The return path isthrough body fluid and tissue generally straight from the Tip electrodein the right ventricle back up to the pacemaker case or housing. Thisforms an enclosed area which can be measured from patient X-rays insquare centimeters. The average loop area is 200 to 225 squarecentimeters. This is an average and is subject to great statisticalvariation. For example, in a large adult patient with an abdominalimplant, the implanted loop area is much larger (greater than 450 squarecentimeters).

Relating now to the specific case of MRI, the magnetic gradient fieldswould be induced through enclosed loop areas. However, the pulsed RFfields, which are generated by the body coil, would be primarily inducedinto the lead system by antenna action.

There are a number of potential problems with MRI, including:

-   -   (1) Closure of the pacemaker reed switch. A pacemaker reed        switch, which can also be a Hall Effect device, is designed to        detect a permanent magnet held close to the patient's chest.        This magnet placement allows a physician or even the patient to        put the implantable medical device into what is known as the        “magnet mode response.” The “magnet mode response” varies from        one manufacturer to another, however, in general, this puts the        pacemaker into a fixed rate or asynchronous pacing mode. This is        normally done for short times and is very useful for diagnostic        and clinical purposes. However, in some cases, when a pacemaker        is brought into the bore or close to the MRI scanner, the MRI        static field can make the pacemaker's internal reed switch        close, which puts the pacemaker into a fixed rate or        asynchronous pacing mode. Worse yet, the reed switch may bounce        or oscillate. Asynchronous pacing may compete with the patient's        underlying cardiac rhythm. This is one reason why patients have        generally been advised not to undergo MRI. Fixed rate or        asynchronous pacing for most patients is not an issue. However,        in patients with unstable conditions, such as myocardial        ischemia, there is a substantial risk for ventricular        fibrillation during asynchronous pacing. In most modern        pacemakers the magnetic reed switch (or Hall Effect device)        function is programmable. If the magnetic reed switch response        is switched off, then synchronous pacing is still possible even        in strong magnetic fields. The possibility to open and re-close        the reed switch in the main magnetic field by the gradient field        cannot be excluded. However, it is generally felt that the reed        switch will remain closed due to the powerful static magnetic        field. It is theoretically possible for certain reed switch        orientations at the gradient field to be capable of repeatedly        closing and re-opening the reed switch.    -   (2) Reed switch damage. Direct damage to the reed switch is        theoretically possible, but has not been reported in any of the        known literature. In an article written by Roger Christoph        Luchinger of Zurich, he reports on testing in which reed        switches were exposed to the static magnetic field of MRI        equipment. After extended exposure to these static magnetic        fields, the reed switches functioned normally at close to the        same field strength as before the test.    -   (3) Pacemaker displacement. Some parts of pacemakers, such as        the batteries and reed switch, contain ferrous magnetic        materials and are thus subject to mechanical forces during MRI.        Pacemaker displacement may occur in response to magnetic force        or magnetic torque. There are several recent reports on modern        pacemakers and ICDs that force and torque are not of concern for        MRI systems up to 3 Tesla.    -   (4) Radio frequency field. At the frequencies of interest in        MRI, RF energy can be absorbed and converted to heat. The power        deposited by RF pulses during MRI is complex and is dependent        upon the power (Specific Absorption Rate (SAR) Level) and        duration of the RF pulse, the transmitted frequency, the number        of RF pulses applied per unit time, and the type of        configuration of the RF transmitter coil used. The amount of        heating also depends upon the volume of tissue imaged, the        electrical resistivity of tissue and the configuration of the        anatomical region imaged. There are also a number of other        variables that depend on the placement in the human body of the        AIMD and its associated lead(s). For example, it will make a        difference how much current is induced into a pacemaker lead        system as to whether it is a left or right pectoral implant. In        addition, the routing of the lead and the lead length are also        very critical as to the amount of induced current and heating        that would occur. Also, distal Tip design is very important as        the distal Tip itself can act as its own antenna wherein eddy        currents can create heating. The cause of heating in an MRI        environment is twofold: (a) RF field coupling to the lead can        occur which induces significant local heating; and (b) currents        induced between the distal Tip and tissue during MRI RF pulse        transmission sequences can cause local Ohms Law heating in        tissue next to the distal Tip electrode of the implanted lead.        The RF field of an MRI scanner can produce enough energy to        induce lead currents sufficient to destroy some of the adjacent        myocardial tissue. Tissue ablation has also been observed. The        effects of this heating are not readily detectable by monitoring        during the MRI. Indications that heating has occurred would        include an increase in pacing threshold, venous ablation, Larynx        or esophageal ablation, myocardial perforation and lead        penetration, or even arrhythmias caused by scar tissue. Such        long term heating effects of MRI have not been well studied yet        for all types of AIMD lead geometries. There can also be        localized heating problems associated with various types of        electrodes in addition to Tip electrodes. This includes Ring        electrodes or PAD electrodes. Ring electrodes are commonly used        with a wide variety of implanted devices including cardiac        pacemakers, and neurostimulators, and the like. PAD electrodes        are very common in neurostimulator applications. For example,        spinal cord stimulators or deep brain stimulators can include a        plurality of PAD electrodes to make contact with nerve tissue. A        good example of this also occurs in a cochlear implant. In a        typical cochlear implant there would be sixteen Ring electrodes        that the position places by pushing the electrode up into the        cochlea. Several of these Ring electrodes make contact with        auditory nerves.    -   (5) Alterations of pacing rate due to the applied radio        frequency field. It has been observed that the RF field may        induce undesirable fast pacing (QRS complex) rates. There are        various mechanisms which have been proposed to explain rapid        pacing: direct tissue stimulation, interference with pacemaker        electronics or pacemaker reprogramming (or reset). In all of        these cases, it is very desirable to raise the lead system        impedance (at the MRI RF pulsed frequency) to make an EMI filter        feedthrough capacitor more effective and thereby provide a        higher degree of protection to AIMD electronics. This will make        alterations in pacemaker pacing rate and/or pacemaker        reprogramming much more unlikely.    -   (6) Time-varying magnetic gradient fields. The contribution of        the time-varying gradient to the total strength of the MRI        magnetic field is negligible, however, pacemaker systems could        be affected because these fields are rapidly applied and        removed. The time rate of change of the magnetic field is        directly related to how much electromagnetic force and hence        current can be induced into a lead system. Luchinger reports        that even using today's gradient systems with a time-varying        field up to 50 Tesla per second, the induced currents are likely        to stay below the biological thresholds for cardiac        fibrillation. A theoretical upper limit for the induced voltage        by the time-varying magnetic gradient field is 20 volts. Such a        voltage during more than 0.1 milliseconds could be enough energy        to directly pace the heart.    -   (7) Heating. Currents induced by time-varying magnetic gradient        fields may lead to local heating. Researchers feel that the        calculated heating effect of the gradient field is much less as        compared to that caused by the RF field and therefore for the        purposes herein may be neglected.

There are additional problems possible with implantable cardioverterdefibrillators (ICDs). ICDs use different and larger batteries whichcould cause higher magnetic forces. The programmable sensitivity in ICDsis normally much higher (more sensitive) than it is for pacemakers,therefore, ICDs may falsely detect a ventricular tachyarrhythmia andinappropriately deliver therapy. In this case, therapy might includeanti-tachycardia pacing, cardio version or defibrillation (high voltageshock) therapies. MRI magnetic fields may prevent detection of adangerous ventricular arrhythmia or fibrillation. There can also beheating problems of ICD leads which are expected to be comparable tothose of pacemaker leads. Ablation of vascular walls is another concern.Fortunately, ICDs have a sort of built-in fail-safe mechanism. That is,during an MRI procedure, if they inadvertently sense the MRI fields as adangerous ventricular arrhythmia, the ICD will attempt to charge up anddeliver a high voltage shock. However, there is a transformer containedwithin the ICD that is necessary to function in order to charge up thehigh-energy storage capacitor contained within the ICD. In the presenceof the main static field of the MRI the core of this transformer tendsto saturate thereby preventing the high voltage capacitor from chargingup. This makes it highly unlikely that an ICD patient undergoing an MRIwould receive an inappropriate high voltage shock therapy. While ICDscannot charge during MRI due to the saturation of their ferro-magnetictransformers, the battery will be effectively shorted and lose life.This is a highly undesirable condition.

In summary, there are a number of studies that have shown that MRIpatients with active implantable medical devices, such as cardiacpacemakers, can be at risk for potential hazardous effects. However,there are a number of reports in the literature that MRI can be safe forimaging of pacemaker patients when a number of precautions are taken(only when an MRI is thought to be an absolute diagnostic necessity).While these anecdotal reports are of interest, however, they arecertainly not scientifically convincing that all MRI can be safe. Aspreviously mentioned, just variations in the pacemaker lead length cansignificantly affect how much heat is generated. From the layman's pointof view, this can be easily explained by observing the typical length ofthe antenna on a cellular telephone compared to the vertical rod antennamore common on older automobiles. The relatively short antenna on thecell phone is designed to efficiently couple with the very highfrequency wavelengths (approximately 950 MHz) of cellular telephonesignals. In a typical AM and FM radio in an automobile, these wavelengthsignals would not efficiently couple to the relatively short antenna ofa cell phone. This is why the antenna on the automobile is relativelylonger. An analogous situation exists with an AIMD patient in an MRIsystem. If one assumes, for example, a 3.0 Tesla hydrogen MRI system,which would have an RF pulsed frequency of 128 MHz, there are certainimplanted lead lengths that would couple efficiently as fractions of the128 MHz wavelength. It is typical that a hospital will maintain aninventory of various leads and that the implanting physician will make aselection depending on the size of the patient, implant location andother factors. Accordingly, the implanted or effective lead length canvary considerably. There are certain implanted lead lengths that just donot couple efficiently with the MRI frequency and there are others thatwould couple very efficiently and thereby produce the worst case forheating.

The effect of an MRI system on the function of pacemakers, ICDs andneurostimulators depends on various factors, including the strength ofthe static magnetic field, the pulse sequence (gradient and RF fieldused), the anatomic region being imaged, and many other factors. Furthercomplicating this is the fact that each patient's condition andphysiology is different and each manufacturer's pacemaker and ICDdesigns also are designed and behave differently. Most experts stillconclude that MRI for the pacemaker patient should not be consideredsafe. Paradoxically, this also does not mean that the patient should notreceive MRI. The physician must make an evaluation given the pacemakerpatient's condition and weigh the potential risks of MRI against thebenefits of this powerful diagnostic tool. As MRI technology progresses,including higher field gradient changes over time applied to thinnertissue slices at more rapid imagery, the situation will continue toevolve and become more complex. An example of this paradox is apacemaker patient who is suspected to have a cancer of the lung. RFablation treatment of such a tumor may require stereotactic imaging onlymade possible through real time fine focus MRI. With the patient's lifeliterally at risk, the physician with patient informed consent may makethe decision to perform MRI in spite of all of the previously describedattendant risks to the pacemaker system.

Insulin drug pump systems do not seem to be of a major current concerndue to the fact that they have no significant antenna components (suchas implanted leads). However, some implantable pumps work onmagneto-peristaltic systems, and must be deactivated prior to MRI. Thereare newer (unreleased) systems that would be based on solenoid systemswhich will have similar problems.

It is clear that MRI will continue to be used in patients with activeimplantable medical devices. Accordingly, there is a need for AIMDsystem and/or circuit protection devices which will improve the immunityof active implantable medical device systems to diagnostic proceduressuch as MRI.

As one can see, many of the undesirable effects in an implanted leadsystem from MRI and other medical diagnostic procedures are related toundesirable induced currents in the lead system and/or its distal Tip(or Ring). This can lead to overheating either in the lead or at thebody tissue at the distal Tip. For a pacemaker application, thesecurrents can also directly stimulate the heart into sometimes dangerousarrhythmias.

Accordingly, there is a need for a novel resonant tank or bandstopfilter assembly which can be placed at various locations along theactive implantable medical device lead system, which also preventscurrent from circulating at selected frequencies of the medicaltherapeutic device. Preferably, such novel tank filters would bedesigned to resonate at 64 MHz for use in a hydrogen MRI systemoperating at 1.5 Tesla (or 128 MHz for a hydrogen 3 Tesla system). Thepresent invention fulfills these needs and provides other relatedadvantages.

SUMMARY OF THE INVENTION

The present invention comprises resonant tank circuits/bandstop filtersto be placed at one or more locations along the active implantablemedical device (AIMD) lead system, including its distal Tip. Thesebandstop filters prevent current from circulating at selectedfrequencies of the medical therapeutic device. For example, for an MRIsystem operating at 1.5 Tesla, the pulse RF frequency is 64 MHz. Thenovel bandstop filters of the present invention can be designed toresonate at 64 MHz and thus create an open circuit in the implanted leadsystem at that selected frequency. For example, the bandstop filter ofthe present invention, when placed at the distal Tip electrode, willprevent currents from flowing through the distal Tip electrode, preventcurrents from flowing in the implanted leads and also prevent currentsfrom flowing into body tissue. It will be apparent to those skilled inthe art that all of the embodiments described herein are equallyapplicable to a wide range of other active implantable medical devices,including deep brain stimulators, spinal cord stimulators, cochlearimplants, ventricular assist devices, artificial hearts, drug pumps, andthe like. The present invention fulfills all of the needs regardingreduction or elimination of undesirable currents and associated heatingin implanted lead systems.

Electrically engineering a capacitor in parallel with an inductor isknown as a tank filter. It is also well known that when the tank filteris at its resonant frequency, it will present a very high impedance.This is a basic principle of all radio receivers. In fact, multiple tankfilters are often used to improve the selectivity of a radio receiver.One can adjust the resonant frequency of the tank circuit by eitheradjusting the capacitor value or the inductor value or both. Sincemedical diagnostic equipment which is capable of producing very largefields operates at discrete frequencies, this is an ideal situation fora specific tank or bandstop filter. Bandstop filters are more efficientfor eliminating one single frequency than broadband filters. Because thebandstop filter is targeted to provide attenuation to induced RF currentat this one selected center frequency or across a range of frequencies,it can be much smaller and volumetrically efficient suitable forincorporation into an implantable medical device. In addition, the wayMRI RF pulsed fields couple with lead systems, various loops andassociated loop currents result along various sections of the implantedlead. For example, at the distal Tip electrode of a cardiac pacemaker,direct electromagnetic forces (EMF) can be produced which result incurrent loops through the distal Tip electrode and into the associatedmyocardial tissue. This current system is largely decoupled from thecurrents that are induced near the active implantable medical device,for example, near the cardiac pacemaker. There the MRI may set up aseparate loop with its associated currents. Accordingly, one or morebandstop filters may be required to completely control all of thevarious induced EMI and associated currents in an implantable leadsystem.

The present invention which resides in bandstop filters is also designedto work in concert with the EMI filter which is typically used at thepoint of lead ingress and egress of the active implantable medicaldevice. For example, see U.S. Pat. No. 5,333,095;U.S. Pat. No.6,999,818; U.S. Pat. No. 7,765,005; US 2007/0083244-A1; the contents ofall being incorporated herein by reference. All of these patentdocuments describe novel inductor capacitor combinations for low passEMI filter circuits. It is of particular interest that by increasing thenumber of circuit elements, one can reduce the overall capacitance valuewhich is at the input to the implantable medical device. It is importantto reduce the capacitance value to raise the input impedance of theactive implantable medical device such that this also reduces the amountof current that would flow in lead systems associated with medicalprocedures such as MRI.

In one embodiment, an implantable lead comprises at least one bandstopfilter comprising at least a portion of the lead, for attenuatingcurrent flow through the lead at a selected center frequency or across arange of frequencies. The bandstop filter comprises a capacitance inparallel with an inductance, wherein values of capacitance andinductance are selected such that the bandstop filter is resonant at theselected center frequency. The inductance is inherently derived from thelead's material of construction or structure, and the capacitance isinherently derived from the lead's material of construction orstructure. The bandstop filter may be disposed at, within or adjacent toa distal electrode of the lead. Moreover, the bandstop filter may beintegrated into a selected tip electrode or a Ring electrode. Theoverall Q of the bandstop filter may also be lowered to attenuatecurrent flow through the lead across a range of selected frequencies.The range of frequencies preferably includes a plurality of MRI RFpulsed frequencies.

The overall Q of the bandstop filter is typically selected to balanceimpedance at the selected center frequency versus frequency bandwidthcharacteristics. A Q_(i) of the inductance is provided to be relativelyhigh and a Q_(c) of the capacitance is provided to be relatively low toreduce the overall Q of the bandstop filter. The Q_(i) of the inductanceis provided to be relatively high by lowering the resistive loss of theinductance. The Q_(c) of the capacitance is relatively lowered byraising the equivalent series resistance of the capacitance. The Q_(i)of the inductance element and the Q_(c) of the capacitance element maybe selected such that the overall Q of the bandstop filter attenuatescurrent flow through the lead at the selected center frequency or acrossthe range of frequencies. The Q_(i) of the inductance may also berelatively lowered and the Q_(c) of the capacitance may be relativelyincreased to reduce the overall Q of the bandstop filter.

The novel lead of the present invention finds particular applicationwith active implantable medical devices (AIMDs), such as cochlearimplants, piezoelectric soundbridge transducers, neurostimulators, brainstimulators, cardiac pacemakers, ventricular assist devices, artificialhearts, drug pumps, bone growth stimulators, bone fusion stimulators,urinary incontinence devices, pain relief spinal cord stimulators,anti-tremor stimulators, gastric stimulators, implantable cardioverterdefibrillators, congestive heart failure devices, neuromodulators, andthe like.

The bandstop filter may comprise at least a part of a coiled or spiralinductor portion of the lead. In this case, the capacitance comprisesparasitic capacitance between adjacent turns of said inductor portion.

Other features and advantages of the present invention will becomeapparent from the following more detailed description, taken inconjunction with the accompanying drawings which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a wire-formed diagram of a generic human body showing a numberof active implantable medical devices (AIMDs);

FIG. 2 is a perspective and somewhat schematic view of a prior artactive implantable medical device (AIMD) including a lead directed tothe heart of a patient;

FIG. 3 is an enlarged sectional view taken generally along the line 3-3of FIG. 2;

FIG. 4 is a view taken generally along the line 4-4 of FIG. 3;

FIG. 5 is a perspective/isometric view of a prior art rectangularquadpolar feedthrough capacitor of the type shown in FIGS. 3 and 4;

FIG. 6 is sectional view taken generally along the line 6-6 of FIG. 5;

FIG. 7 is a sectional view taken generally along the line 7-7 of FIG. 5;

FIG. 8 is a diagram of a unipolar active implantable medical device;

FIG. 9 is a diagram similar to FIG. 8, illustrating a bipolar AIMDsystem;

FIG. 10 is a diagram similar to FIGS. 8 and 9, illustrating a bipolarlead system with a distal Tip and Ring, typically used in a cardiacpacemaker;

FIG. 11 is a schematic diagram showing a parallel combination of aninductor L and a capacitor C placed in series with the lead systems ofFIGS. 8-10;

FIG. 12 is a chart illustrating calculation of frequency of resonancefor the parallel tank circuit of FIG. 11;

FIG. 13 is a graph showing impedance versus frequency for the paralleltank bandstop circuit of FIG. 11;

FIG. 14 is an equation for the impedance of an inductor in parallel witha capacitor;

FIG. 15 is a chart illustrating reactance equations for the inductor andthe capacitor of the parallel tank circuit of FIG. 11;

FIG. 16 is a schematic diagram illustrating the parallel tank circuit ofFIG. 11, except in this case the inductor and the capacitor have seriesresistive losses;

FIG. 17 is a diagram similar to FIG. 8, illustrating the tankcircuit/bandstop filter added near a distal electrode;

FIG. 18 is a schematic representation of the novel bandstop tank filterof the present invention, using switches to illustrate its function atvarious frequencies;

FIG. 19 is a schematic diagram similar to FIG. 18, illustrating the lowfrequency model of the bandstop filter;

FIG. 20 is a schematic diagram similar to FIGS. 18 and 19, illustratingthe model of the bandstop filter of the present invention at itsresonant frequency;

FIG. 21 is a schematic diagram similar to FIGS. 18-20, illustrating amodel of the bandstop filter at high frequencies well above the resonantfrequency;

FIG. 22 is a decision tree block diagram illustrating a process fordesigning the bandstop filters of the present invention;

FIG. 23 is graph of insertion loss versus frequency for bandstop filtershaving high Q inductors and differing quality “Q” factors;

FIG. 24 is a tracing of an exemplary patient x-ray showing an implantedpacemaker and cardioverter defibrillator and corresponding lead system;

FIG. 25 is a line drawings of an exemplary patent cardiac x-ray of abi-ventricular lead system;

FIG. 26 illustrates a bipolar cardiac pacemaker lead showing the distalTip and the distal Ring electrodes;

FIG. 27 is an enlarged, fragmented schematic illustration of the areaillustrated by the line 27-27 in FIG. 26; and

FIG. 28 is a schematic illustration of the area 28-28 from FIG. 26,illustrating how inductive coil parasitic capacitance is utilized toform the bandstop filters of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a wire formed diagram of a generic human body and illustratesof various types of active implantable medical devices 100 that arecurrently in use. 100A is a family of implantable hearing devices whichcan include the group of cochlear implants, piezoelectric sound bridgetransducers and the like. 1008 includes an entire variety ofneurostimulators and brain stimulators. Neurostimulators are used tostimulate the Vagus nerve, for example, to treat epilepsy, obesity anddepression. Brain stimulators are similar to a pacemaker-like device andinclude electrodes implanted deep into the brain for sensing the onsetof the seizure and also providing electrical stimulation to brain tissueto prevent the seizure from actually happening. 100C shows a cardiacpacemaker which is well-known in the art. 100D includes the family ofleft ventricular assist devices (LVAD's), and artificial hearts,including the recently introduced artificial heart known as the Abiocor.100E includes an entire family of drug pumps which can be used fordispensing of insulin, chemotherapy drugs, pain medications and thelike. Insulin pumps are evolving from passive devices to ones that havesensors and closed loop systems. That is, real time monitoring of bloodsugar levels will occur. These devices tend to be more sensitive to EMIthan passive pumps that have no sense circuitry or externally implantedleads. 100F includes a variety of implantable bone growth stimulatorsfor rapid healing of fractures. 100G includes urinary incontinencedevices. 100H includes the family of pain relief spinal cord stimulatorsand anti-tremor stimulators. 100H also includes an entire family ofother types of neurostimulators used to block pain. 100I includes afamily of implantable cardioverter defibrillators (ICD) devices and alsoincludes the family of congestive heart failure devices (CHF). This isalso known in the art as cardio resynchronization therapy devices,otherwise known as CRT devices.

It will be appreciated for purposes of the present invention that theactive implantable medical devices and associated lead systems to whichthe present invention applies are typically permanently orsemi-permanently implanted within the body. The lead systems for suchAIMDs cannot be easily removed temporarily for purposes of conducting anMRI scan on a patient. In contrast, probes and some forms of cathetersmay be temporarily inserted into the body, but are quite easy towithdraw during an MRI scan and thus obviate the need for the safeguardsprovided by the present invention.

Referring now to FIG. 2, a prior art active implantable medical device(AIMD) 100 is illustrated. In general, the AIMD 100 could, for example,be a cardiac pacemaker 100C which is enclosed by a titanium housing 102as indicated. The titanium housing is hermetically sealed, however thereis a point where lead wires 104 must ingress and egress the hermeticseal. This is accomplished by providing a hermetic terminal assembly106. Hermetic terminal assemblies are well known and generally consistof a ferrule 108 which is laser welded to the titanium housing 102 ofthe AIMD 100. The hermetic terminal assembly 106 with its associated EMIfilter is better shown in FIG. 3. Referring once again to FIG. 2, fourleads are shown consisting of lead wire pair 104 a and 104 b and leadwire pair 104 c and 104 d. This is typical of what's known as a dualchamber bipolar cardiac pacemaker.

The IS1 connectors 110 that are designed to plug into the header block112 are low voltage (pacemaker) connectors covered by an ANSI/AAMIstandard IS-1. Higher voltage devices, such as implantable cardioverterdefibrillators, are covered by a standard known as the ANSI/AAMI DF-1.There is a new standard under development which will integrate both highvoltage and low voltage connectors into a new miniature connector seriesknown as the IS-4 series. These connectors are typically routed in apacemaker application down into the right ventricle and right atrium ofthe heart 114. There are also new generation devices that have beenintroduced to the market that couple leads to the outside of the leftventricle. These are known as biventricular devices and are veryeffective in cardiac resynchronization therapy (CRT) and treatingcongestive heart failure (CHF).

Referring once again to FIG. 2, one can see, for example, the bipolarlead wires 104 a and 104 b that could be routed, for example, to thedistal Tip and Ring into the right ventricle. The bipolar lead wires 104c and 104 d could be routed to a distal Tip and Ring in the rightatrium. There is also an RF telemetry pin antenna 116 which is notconnected to the IS-1 or DS-1 connector block. This acts as a short stubantenna for picking up telemetry signals that are transmitted from theoutside of the device 100.

It should also be apparent to those skilled in the art that all of thedescriptions herein are equally applicable to other types of AIMDs.These include implantable cardioverter defibrillators (ICDs),neurostimulators, including deep brain stimulators, spinal cordstimulators, cochlear implants, incontinence stimulators and the like,and drug pumps.

FIG. 3 is an enlarged, fragmented cross-sectional view taken generallyalong line 3-3 of FIG. 2. Here one can see in cross-section the RFtelemetry pin 116 and the bipolar leads 104 a and 104 c which would berouted to the cardiac chambers by connecting these leads to the internalconnectors 118 of the IS-1 header block 112 (FIG. 2). These connectorsare designed to receive the plug 110 which allows the physicians tothread leads through the venous system down into the appropriatechambers of the heart 114. It will be obvious to those skilled in theart that tunneling of deep brain electrodes or neurostimulators areequivalent.

Referring back to FIG. 3, one can see a prior art feedthrough capacitor120 which has been bonded to the hermetic terminal assembly 106. Thesefeedthrough capacitors are well known in the art and are described andillustrated in U.S. Pat. Nos. 5,333,095, 5,751,539, 5,905,627,5,959,829, 5,973,906, 5,978,204, 6,008,980, 6,159,560, 6,275,369,6,424,234, 6,456,481, 6,473,291, 6,529,103, 6,566,978, 6,567,259,6,643,903, 6,675,779, 6,765,780 and 6,882,248. In this case, arectangular quadpolar feedthrough capacitor 120 is illustrated which hasan external metalized termination surface 122. It includes embeddedelectrode plate sets 124 and 126. Electrode plate set 124 is known asthe ground electrode plate set and is terminated at the outside of thecapacitor 120 at the termination surface 122. These ground electrodeplates 124 are electrically and mechanically connected to the ferrule108 of the hermetic terminal assembly 106 using a thermosettingconductive polyimide or equivalent material 128 (equivalent materialswill include solders, brazes, conductive epoxies and the like). In turn,the hermetic seal terminal assembly 106 is designed to have its titaniumferrule 108 laser welded 130 to the overall housing 102 of the AIMD 100.This forms a continuous hermetic seal thereby preventing body fluidsfrom penetrating into and causing damage to the electronics of the AIMD.

It is also essential that the leads 104 and insulator 136 behermetically sealed, such as by the gold brazes or glass seals132 and134. The gold braze 132 wets from the titanium ferrule 108 to thealumina ceramic insulator 136. In turn, the ceramic alumina insulator136 is also gold brazed at 134 to each of the leads 104. The RFtelemetry pin 116 is also gold brazed at 138 to the alumina ceramicinsulator 136. It will be obvious to those skilled in the art that thereare a variety of other ways of making such a hermetic terminal. Thiswould include glass sealing the leads into the ferrule directly withoutthe need for the gold brazes.

As shown in FIG. 3, the RF telemetry pin 116 has not been included inthe area of the feedthrough capacitor 120. The reason for this is thefeedthrough capacitor 120 is a very broadband single element EMI filterwhich would eliminate the desirable telemetry frequency.

FIG. 4 is a bottom view taken generally along line 4-4 in FIG. 3. Onecan see the gold braze 132 which completely seals the hermetic terminalinsulator 136 into the overall titanium ferrule 108. One can also seethe overlap of the capacitor attachment materials, shown as athermosetting conductive adhesive 128, which makes contact to the goldbraze 132 that forms the hermetic terminal 106.

FIG. 5 is an isometric view of the feedthrough capacitor 120. As one cansee, the termination surface 122 connects to the capacitor's internalground plate set 124. This is best seen in FIG. 6 where ground plate set124, which is typically silk-screened onto ceramic layers, is broughtout and exposed to the termination surface 122. The capacitor's four(quadpolar) active electrode plate sets 126 are illustrated in FIG. 7.In FIG. 6 one can see that the leads 104 are in non-electricalcommunication with the ground electrode plate set 124. However, in FIG.7 one can see that each one of the leads 104 is in electrical contactwith its corresponding active electrode plate set 126. The amount ofcapacitance is determined by the overlap of the active electrode platearea 126 over the ground electrode plate area. One can increase theamount of capacitance by increasing the area of the active electrodeplate set 126. One can also increase the capacitance by addingadditional layers. In this particular application, we are only showingsix electrode layers: three ground plates 124 and three active electrodeplate sets 126 (FIG. 3). However, 10, 60 or even more than 100 such setscan be placed in parallel thereby greatly increasing the capacitancevalue. The capacitance value is also related to the dielectric thicknessor spacing between the ground electrode set 124 and the active electrodeset 126. Reducing the dielectric thickness increases the capacitancesignificantly while at the same time reducing its voltage rating. Thisgives the designer many degrees of freedom in selecting the capacitancevalue.

In the following description, functionally equivalent elements shown invarious embodiments will often be referred to utilizing the samereference number.

FIG. 8 is a general diagram of a unipolar active implantable medicaldevice system 100. The housing 102 of the active implantable medicaldevice 100 is typically titanium, ceramic, stainless steel or the like.Inside of the device housing are the AIMD electronic circuits. UsuallyAIMDs include a battery, but that is not always the case. For example,for a Bion, it can receive its energy from an external pulsing magneticfield. A lead 104 is routed from the AIMD 100 to a point 140 where it isembedded in or affixed to body tissue. In the case of a spinal cordstimulator 100H, the distal tip 140 could be in the spinal cord. In thecase of a deep brain stimulator 1008, the distal electrode 140 would beplaced deep into the brain, etc. In the case of a cardiac pacemaker100C, the distal electrode 140 would typically be placed in the cardiacright ventricle.

FIG. 9 is very similar to FIG. 8 except that it is a bipolar system. Inthis case, the electric circuit return path is between the two distalelectrodes 140 and 140′. In the case of a cardiac pacemaker 100C, thiswould be known as a bipolar lead system with one of the electrodes knownas the distal Tip electrode 142 and the other electrode which wouldfloat in the blood pool known as the Ring electrode 144 (see FIG. 10).In contrast, the electrical return path in FIG. 8 is between the distalelectrode 140 through body tissue to the conductive housing 102 of theimplantable medical device 100.

FIG. 10 illustrates a bipolar lead system with a distal Tip electrode142 and Ring electrode 144 typically as used in a cardiac pacemaker100C. In all of these applications, the patient could be exposed to thefields of an MRI scanner or other powerful emitter used during a medicaldiagnostic procedure. Currents that are directly induced in the leadsystem 104 can cause heating by I²R losses in the lead system or byheating caused by current flowing in body tissue. If these currentsbecome excessive, the associated heating can cause damage or evendestructive ablation to body tissue.

The distal Tip electrode 142 is designed to be implanted into or affixedto the actual myocardial tissue of the heart. The Ring electrode 144 isdesigned to float in the blood pool. Because the blood is flowing and isthermally conductive, the Ring electrode 144 structure is substantiallycooled. In theory, however, if the lead curves, the Ring electrode 144could also touch and become encapsulated by body tissue. The distal Tipelectrode 142, on the other hand, is always thermally insulated bysurrounding body tissue and can readily heat up due to the RF pulsecurrents of an MRI field.

FIG. 11 is a schematic diagram showing a parallel combination of aninductor L and a capacitor C to be placed in the implantable leadsystems 104 previously described. This combination forms a parallel tankcircuit or bandstop filter 146 which will resonate at a particularfrequency (f_(r)).

FIG. 12 gives the frequency of resonance equation f_(r) for the paralleltank circuit 146 of FIG. 11: where f_(r) is the frequency of resonancein hertz, L is the inductance in henries and C is the capacitance infarads. MRI systems vary in static field strength from 0.5 Tesla all theway up to 3 Tesla with newer research machines going much higher. Thisis the force of the main static magnetic field. The frequency of thepulsed RF field associated with MRI is found by multiplying the staticfield in Teslas times 42.45. Accordingly, a 3 Tesla MRI system has apulsed RF field of approximately 128 MHz.

Referring once again to FIG. 11, one can see that if the values of theinductor and the capacitor are selected properly, one could obtain aparallel tank resonant frequency of 128 MHz. For a 1.5 Tesla MRI system,the RF pulse frequency is 64 MHz. Referring to FIG. 12, one can see thecalculations assuming that the inductor value L is equal to onenanohenry. The one nanohenry comes from the fact that given the smallgeometries involved inside of the human body, a very large inductor willnot be possible. This is in addition to the fact that the use of ferritematerials or iron cores for such an inductor are not practical for tworeasons: 1) the static magnetic field from the MRI scanner would alignthe magnetic dipoles (saturate) in such a ferrite and therefore make theinductor ineffective; and 2) the presence of ferrite materials willcause severe MRI image artifacts. What this means is that if one wereimaging the right ventricle of the heart, for example, a fairly largearea of the image would be blacked out or image distorted due to thepresence of these ferrite materials and the way it interacts with theMRI field. It is also important that the inductance value not vary whilein the presence of the main static field.

The relationship between the parallel inductor L and capacitor C is alsovery important. One could use, for example, a very large value ofinductance which would result in a very small value of capacitance to beresonant, for example, at the MRI frequency of 64 MHz. However, using avery high value of inductor results in a high number of turns of verysmall wire. Using a high number of turns of very small diameter wire iscontraindicated for two reasons. The first reason is that the longlength of relatively small diameter wire results in a very high DCresistance for the inductor. This resistance is very undesirable becauselow frequency pacing or neurostimulator pulses would lose energy passingthrough the relatively high series resistance. This is also undesirablewhere the AIMD is sensing biologic signals. For example, in the case ofa pacemaker or deep brain stimulator, continuous sensing of lowfrequency biological signals is required. Too much series resistance ina lead system will attenuate such signals thereby making the AIMD lessefficient. Accordingly, it is a preferred feature of the presentinvention that a relatively large value of capacitance will be used inparallel with a relatively small value of inductance, for example,employing highly volumetrically efficient ceramic dielectric capacitorsthat can create a great deal of capacitance in a very small space.

It should be also noted that below resonance, particularly at very lowfrequencies, the current in the parallel L-C band width stop filterpasses through the inductor element. Accordingly, it is important thatthe parasitic resistance of the inductor element be quite low.Conversely, at very low frequencies, no current passes through thecapacitor element. At high frequencies, the reactance of the capacitorelement drops to a very low value. However, as there is no case where itis actually desirable to have high frequencies pass through the tankfilter, the parasitic resistive loss of the capacitor is notparticularly important. This is also known as the capacitor's equivalentseries resistance (ESR). A component of capacitor ESR is the dissipationfactor of the capacitor (a low frequency phenomena). Off of resonance,it is not particularly important how high the capacitor's dissipationfactor or overall ESR is when used as a component of a parallel tankcircuit 146 as described herein. Accordingly, an air wound inductor isthe ideal choice because it is not affected by MRI signals or fields.Because of the space limitations, however, the inductor will not be veryvolumetrically efficient. For this reason, it is preferable to keep theinductance value relatively low (in the order of 1 to 100 nanohenries).

Referring once again to FIG. 12, one can see the calculations forcapacitance by algebraically solving the resonant frequency f_(r)equation shown for C. Assuming an inductance value of one nanohenry, onecan see that 6 nano-farads of capacitance would be required. Sixnano-farads of capacitance is a relatively high value of capacitance.However, ceramic dielectrics that provide a very high dielectricconstant are well known in the art and are very volumetricallyefficient. They can also be made of biocompatible materials making theman ideal choice for use in the present invention.

FIG. 13 is a graph showing impedance versus frequency for the paralleltank, bandstop filter circuit 146 of FIG. 11. As one can see, usingideal circuit components, the impedance measured between points A and Bfor the parallel tank circuit 146 shown in FIG. 11 is very low (zero)until one approaches the resonant frequency f_(r). At the frequency ofresonance, these ideal components combine together to look like a veryhigh or, ideally, an infinite impedance. The reason for this comes fromthe denominator of the equation Z_(ab) for the impedance for theinductor in parallel with the capacitor shown as FIG. 14. When theinductive reactance is equal to the capacitive reactance, the twoimaginary vectors cancel each other and go to zero. Referring to theequations in FIGS. 14 and 15, one can see in the impedance equation forZ_(ab), that a zero will appear in the denominator when X_(L)=X_(C).This has the effect of making the impedance approach infinity as thedenominator approaches zero. As a practical matter, one does not reallyachieve an infinite impedance. However, tests have shown that severalhundred ohms can be realized which offers a great deal of attenuationand protection to RF pulsed currents from MRI. What this means is thatat one particular unique frequency, the impedance between points A and Bin FIG. 11 will appear very high (analogous to opening a switch).Accordingly, it would be possible, for example, in the case of a cardiacpacemaker, to design the cardiac pacemaker for compatibility with onesingle popular MRI system. For example, in the AIMD patient literatureand physician manual it could be noted that the pacemaker lead systemhas been designed to be compatible with 3 Tesla MRI systems.Accordingly, with this particular device, a distal Tip bandstop filter146 would be incorporated where the L and the C values have beencarefully selected to be resonant at 128 MHz, presenting a high oralmost infinite impedance at the MRI pulse frequency.

FIG. 16 is a schematic drawing of the parallel tank circuit 146 of FIG.11, except in this case the inductor L and the capacitor C are notideal. That is, the capacitor C has its own internal resistance R_(C),which is otherwise known in the industry as dissipation factor orequivalent series resistance (ESR). The inductor L also has a resistanceR_(L). For those that are experienced in passive components, one wouldrealize that the inductor L would also have some parallel capacitance.This parasitic capacitance comes from the capacitance associated withadjacent turns. However, the inductance value contemplated is so lowthat one can assume that at MRI pulse frequencies, the inductor'sparallel capacitance is negligible. One could also state that thecapacitor C also has some internal inductance which would appear inseries. However, the novel capacitors described below are very small orcoaxial and have negligible series inductance. Accordingly, the circuitshown in FIG. 16 is a very good approximation model for the novelparallel tank circuits 146 as described herein.

This is best understood by looking at the FIG. 16 circuit 146 at thefrequency extremes. At very low frequency, the inductor reactanceequation is X_(L)=2πfL (reference FIG. 15). When the frequency f isclose to zero (DC), this means that the inductor looks like a shortcircuit. It is generally the case that biologic signals are lowfrequency, typically between 10 Hz and 1000 Hz. For example, in acardiac pacemaker 100C, all of the frequencies of interest appearbetween 10 Hz and 1000 Hz. At these low frequencies, the inductivereactance X_(L) will be very close to zero ohms. Over this range, on theother hand, the capacitive reactance X_(C) which has the equationX_(C)=1/(2πfc) will look like an infinite or open circuit (referenceFIG. 15). As such, at low frequencies, the impedance between points Aand B in FIG. 16 will equal to R_(L). Accordingly, the resistance of theinductor (R_(L)) should be kept as small as possible to minimizeattenuation of biologic signals or attenuation of stimulation pulses tobody tissues. This will allow biologic signals to pass through thebandstop filter 146 freely. It also indicates that the amount ofcapacitive loss R_(C) is not particularly important. As a matter offact, it would be desirable if that loss were fairly high so as to notfreely pass very high frequency signals (such as undesirable EMI fromcellular phones). It is also desirable to have the Q of the circuitshown in FIG. 16 relatively low so that the bandstop frequency bandwidthcan be a little wider. In other words, in a preferred embodiment, itwould be possible to have a bandstop wide enough to block both 64 MHzand 128 MHz frequencies thereby making the medical device compatible foruse in both 1.5 Tesla and 3 Tesla MRI systems.

FIG. 17 is a drawing of the unipolar AIMD lead system, previously shownin FIG. 8, with the bandstop filter 146 of the present invention addednear the distal electrode 140. As previously described, the presence ofthe tank circuit 146 will present a very high impedance at one or morespecific MRI RF pulse frequencies. This will prevent currents fromcirculating through the distal electrode 140 into body tissue at thisselected frequency(s). This will provide a very high degree of importantprotection to the patient so that overheating does not cause tissuedamage.

FIG. 18 is a representation of the novel bandstop tank filter 146 usingswitches that open and close at various frequencies to illustrate itsfunction. Inductor L has been replaced with a switch S_(L). When theimpedance of the inductor is quite low, the switch S_(L) will be closed.When the impedance or inductive reactance of the inductor is high, theswitch S_(L) will be shown open. There is a corresponding analogy forthe capacitor element C. When the capacitive reactance looks like a verylow impedance, the capacitor switch S_(C) will be shown closed. When thecapacitive reactance is shown as a very high impedance, the switch S_(C)will be shown open. This analogy is best understood by referring toFIGS. 19, 20 and 21.

FIG. 19 is the low frequency model of the bandstop filter 146. At lowfrequencies, capacitors tend to look like open circuits and inductorstend to look like short circuits. Accordingly, switch S_(L) is closedand switch S_(C) is open. This is an indication that at frequenciesbelow the resonant frequency of the bandstop filter 146 that currentswill flow only through the inductor element and its correspondingresistance R_(L). This is an important consideration for the presentinvention that low frequency biological signals not be attenuated. Forexample, in a cardiac pacemaker, frequencies of interest generally fallbetween 10 Hz and 1000 Hz. Pacemaker pacing pulses fall within thisgeneral frequency range. In addition, the implantable medical device isalso sensing biological frequencies in the same frequency range.Accordingly, such signals must be able to flow readily through thebandstop filter's inductor element. A great deal of attention should bepaid to the inductor design so that it has a very high quality factor(Q_(i)) and a very low value of parasitic series resistance R_(L).

FIG. 20 is a model of the novel bandstop filter 146 at its resonantfrequency. By definition, when a parallel tank circuit is at resonance,it presents a very high impedance to the overall circuit. Accordingly,both switches S_(L) and S_(C) are shown open. For example, this is howthe bandstop filter 146 prevents the flow of MRI currents throughpacemaker leads and/or into body tissue at a selected MRI RF pulsedfrequency.

FIG. 21 is a model of the bandstop filter 146 at high frequency. At highfrequencies, inductors tend to look like open circuits. Accordingly,switch S_(L) is shown open. At high frequencies, ideal capacitors tendto look like short circuits, hence switch S_(C) is closed. It should benoted that real capacitors are not ideal and tend to degrade inperformance at high frequency. This is due to the capacitor's equivalentseries inductance and equivalent series resistance. Fortunately, for thepresent invention, it is not important how lossy (resistive) thecapacitor element C gets at high frequency. This will only serve toattenuate unwanted electromagnetic interference from flowing in the leadsystem. Accordingly, in terms of biological signals, the equivalentseries resistance R_(C) and resulting quality factor of the capacitorelement C is not nearly as important as the quality factor of theinductor element L. The equation for inductive reactance (X_(L)) isgiven in FIG. 15. The capacitor reactance equation (X_(C)) is also givenin FIG. 15. As one can see, when one inserts zero or infinity for thefrequency, one derives the fact that at very low frequencies inductorstend to look like short circuits and capacitors tend to look like opencircuits. By inserting a very high frequency into the same equations,one can see that at very high frequency ideal inductors look like aninfinite or open impedance and ideal capacitors look like a very low orshort circuit impedance.

FIG. 22 is a decision tree block diagram that better illustrates thedesign process herein. Block 148 is an initial decision step thedesigner must make. For illustrative purposes, we will start with avalue of capacitance that is convenient. This value of capacitance isgenerally going to relate to the amount of space available in the AIMDlead system and other factors. These values for practical purposesgenerally range in capacitance value from a few tens of picofarads up toabout 10,000 picofarads. This puts practical boundaries on the amount ofcapacitance that can be effectively packaged within the scope of thepresent invention. However, that is not intended to limit the generalprinciples of the present invention, but just describe a preferredembodiment. Accordingly, in the preferred embodiment, one will selectcapacitance values generally ranging from 100 picofarads up to about4000 picofarads and then solve for a corresponding inductance valuerequired to be self-resonant at the selected telemetry frequency.Referring back to FIG. 22, one makes the decision whether the design wasC first or L first. If one makes a decision to assume a capacitancevalue C first then one is directed to the left to block 150. In block150, one does an assessment of the overall packaging requirements of adistal Tip 142 bandstop filter 146 and then assumes a realizablecapacitance value. So, in decision block 150, we assume a capacitorvalue. We then solve the resonant tank equation f_(r) from FIG. 12 atblock 152 for the required value of inductance (L). We then look at anumber of inductor designs to see if the inductance value is realizablewithin the space, parasitic resistance R_(C), and other constraints ofthe design. If the inductance value is realizable, then we go on toblock 154 and finalize the design. If the inductance value is notrealizable within the physical and practical constraints, then we needto go back to block 150 and assume a new value of capacitance. One maygo around this loop a number of times until one finally comes up with acompatible capacitor and an inductor design. In some cases, one will notbe able to achieve a final design using this alone. In other words, onemay have to use a custom capacitor value or design in order to achieve aresult that meets all of the design criteria. That is, a capacitordesign with high enough internal losses R_(C) and an inductor designwith low internal loss R_(L) such that the bandstop filter 146 has therequired quality factor (Q), that it be small enough in size, that ithave sufficient current and high voltage handling capabilities and thelike. In other words, one has to consider all of the design criteria ingoing through this decision tree.

In the case where one has gone through the left hand decision treeconsisting of blocks 150, 152 and 154 a number of times and keeps comingup with a “no,” then one has to assume a realizable value of inductanceand go to the right hand decision tree starting at block 156. One thenassumes a realizable value of inductance (L) with a low enough seriesresistance for the inductor R_(L) such that it will work and fit intothe design space and guidelines. After one assumes that value ofinductance, one then goes to decision block 158 and solves the equationC in FIG. 12 for the required amount of capacitance. After one finds thedesired amount of capacitance C, one then determines whether that customvalue of capacitance will fit into the design parameters. If thecapacitance value that is determined in step 160 is realizable, then onegoes on and finalizes the design. However, if it is not realizable, thenone can go back up to step 156, assume a different value of L and gothrough the decision tree again. This is done over and over until onefinds combinations of L and C that are practical for the overall design.

For purposes of the present invention, it is possible to use seriesdiscrete inductors or parallel discrete capacitors to achieve the sameoverall result. For example, in the case of the inductor element L, itwould be possible to use two, three or even more (n) individual inductorelements in series. The same is true for the capacitor element thatappears in the parallel tank filter 146. By adding or subtractingcapacitors in parallel, we are also able to adjust the total capacitancethat ends up resonating in parallel with the inductance.

It is also possible to use a single inductive component that hassignificant parasitic capacitance between its adjacent turns. A carefuldesigner using multiple turns could create enough parasitic capacitancesuch that the coil becomes self-resonant at a predetermined frequency.In this case, the predetermined frequency would be the MRI pulsedfrequency.

Efficiency of the overall tank circuit 146 is also measured in terms ofa quality factor, Q, although this factor is defined differently thanthe one previously mentioned for discrete capacitors and inductors. Thecircuit Q is typically expressed using the following equation:

$Q = \frac{f_{r}}{\Delta \; f_{3{dB}}}$

Where f_(r) is the resonance frequency, and Δf_(3dB) shown as points aand b in FIG. 23, is the bandwidth of the bandstop filter 146. Bandwidthis typically taken as the difference between the two measuredfrequencies, f₁ and f₂, at the 3 dB loss points as measured on aninsertion loss chart, and the resonance frequency is the average betweenf₁ and f₂. As can be seen in this relationship, higher Q values resultin a narrower 3 dB bandwidth.

Material and application parameters must be taken into considerationwhen designing tank filters. Most capacitor dielectric materials age1%-5% in capacitance values per decade of time elapsed, which can resultin a shift of the resonance frequency of upwards of 2.5%. In a high-Qfilter, this could result in a significant and detrimental drop in thebandstop filter performance. A lower-Q filter would minimize the effectsof resonance shift and would allow a wider frequency band through thefilter. However, very low Q filters display lower than desirableattenuation behavior at the desired bandstop frequency (see FIG. 23,curve 162). For this reason, the optimum Q for the bandstop filter ofthe present invention will embody a high Q_(i) inductor L and arelatively low Q_(c) capacitor C which will result in a medium Q tankfilter as shown in curve 164 of FIG. 23.

Accordingly, the “Q” or quality factor of the tank circuit is veryimportant. As mentioned, it is desirable to have a very low loss circuitat low frequencies such that the biological signals not be undesirablyattenuated. The quality factor not only determines the loss of thefilter, but also affects its 3 dB bandwidth. If one does a plot of thefilter response curve (Bode plot), the 3 dB bandwidth determines howsharply the filter will rise and fall. With reference to curve 166 ofFIG. 23, for a tank that is resonant at 128 MHz, an ideal response wouldbe one that had infinite attenuation at 128 MHz, but had zeroattenuation at low frequencies below 1 KHz. Obviously, this is notpossible given the space limitations and the realities of the parasiticlosses within components. In other words, it is not possible (other thanat cryogenic temperatures) to build an inductor that has zero internalresistance. On the other hand, it is not possible to build a perfect(ideal) capacitor either. Capacitors have internal resistance known asequivalent series resistance and also have small amounts of inductance.Accordingly, the practical realization of a circuit, to accomplish thepurposes of the present invention, is a challenging one.

The performance of the circuit is directly related to the efficiency ofboth the inductor and the capacitor; the less efficient each componentis, the more heat loss that results, and this can be expressed by theaddition of resistor elements to the ideal circuit diagram. The effectof lower Q in the tank circuit is to broaden the resonance peak aboutthe resonance frequency. By deliberately using a low Q_(c) capacitor,one can broaden the resonance such that a high impedance (highattenuation) is presented at multiple MRI RF frequencies, for example 64MHz and 128 MHz.

Referring again to FIG. 23, one can see curve 164 wherein a lowresistive loss high Q_(i) inductor has been used in combination with arelatively high ESR low Q_(c) capacitor. This has a very desirableeffect in that at very low frequencies, the impedance of the tankcircuit 146 is essentially zero ohms (or zero dB loss). This means thatbiologic frequencies are not undesirably attenuated. However, one cansee that the 3 db bandwidth is much larger. This is desirable as it willblock multiple RF frequencies. As one goes even higher in frequency,curve164 will desirably attenuate other high frequency EMI signals, suchas those from cellular telephones, microwave ovens and the like.Accordingly, it is often desirable that very low loss inductors be usedin combination with relatively high loss (and/or high inductance)capacitors to achieve a medium or lower Q bandstop filter. Againreferring to FIG. 23, one can see that if the Q of the overall circuitor of the individual components becomes too low, then we have a seriousdegradation in the overall attenuation of the bandstop filter at the MRIpulse frequencies. Accordingly, a careful balance between componentdesign and tank circuit Q must be achieved.

Referring once again to FIG. 17, one can also increase the value ofR_(C) by adding a separate discrete component in series with thecapacitor element. For example, one could install a small capacitor chipthat had a very low equivalent series resistance and place it in serieswith a resistor chip. This would be done to deliberately raise the valueof R_(C) in the circuit as shown in FIG. 17. By carefully adjusting thisvalue of R_(C), one could then achieve the ideal curve 164 as shown inFIG. 23.

FIG. 24 is a tracing of an actual patient X-ray. This particular patientrequired both a cardiac pacemaker 100C and an implantable cardioverterdefibrillator 100I. The corresponding implantable lead system 104, asone can see, makes for a very complicated antenna and loop couplingsituation. The reader is referred to the article entitled, “Estimationof Effective Lead Loop Area for Implantable Pulse Generator andImplantable Cardioverter Defibrillators” provided by the AAMI PacemakerEMC Task Force.

Referring again to FIG. 24, one can see that from the pacemaker 100C,there is an electrode in both the right atrium and in the rightventricle. Both these involve a Tip and Ring electrode. In the industry,this is known as a dual chamber bipolar lead system. Accordingly, thebandstop filters 146 of the present invention would need to be placed atleast in the distal tip in the right atrium and the distal tip in theright ventricle from the cardiac pacemaker. One can also see that theimplantable cardioverter defibrillator (ICD) 100I is implanted directlyinto the right ventricle. Its shocking Tip electrode and perhaps itssuper vena cava (SVC) shock coil would also require a bandstop filtersof the present invention so that MRI exposure cannot induce excessivecurrents into the associated lead system (S). Modern implantablecardioverter defibrillators (ICDs) incorporate both pacing andcardioverting (shock) features. Accordingly, it is becoming quite rarefor a patient to have a lead layout as shown in the X-ray of FIG. 24.However, the number of electrodes remains the same. There are also newercombined pacemaker/ICD systems which include biventricular pacemaking(pacing of the left ventricle). These systems can have as many as 9 toeven 12 leads.

FIG. 25 is a line drawing of an actual patient cardiac X-ray of one ofthe newer bi-ventricular lead systems with various types of electrodeTips shown. The new bi-ventricular systems are being used to treatcongestive heart failure, and make it possible to implant leads outsideof the left ventricle. This makes for a very efficient pacing system;however, the implantable lead system 104 is quite complex. When a leadsystem 104, such as those described in FIGS. 8, 9, 10 and 11, areexposed to a time varying electromagnetic field, electric currents canbe induced into such lead systems. For the bi-ventricular system,bandstop filters 146 would be required at each of the three distal Tipsand optionally at Ring and SVC locations.

FIG. 26 illustrates a single chamber bipolar cardiac pacemaker leadshowing the distal Tip 142 and the distal Ring 144 electrodes. This is aspiral wound system where the Ring coil 104 is wrapped around the Tipcoil 104′. There are other types of pacemaker lead systems in whichthese two leads lay parallel to one another (known as a bifilar leadsystem).

FIG. 27 is a schematic illustration of the area 27-27 in FIG. 26. In thearea of the distal Tip 142 and Ring 144 electrodes, bandstop filters 146and 146′ have been placed in series with each of the respective Tip andRing circuits. Accordingly, at MRI pulsed frequencies, an open circuitwill be presented thereby stopping the flow of undesirable RF current.

FIG. 28 is a schematic illustration of the area 28-28 from FIG. 26. Tothe left of the bandstop filters 146 a and 146 b, the implantable leadconductors 104 and 104′ can either be coiled as shown or straight(filer) or the like. By way of background, the individual conductors ofprior art implanted leads are usually not individually insulated. In theprior art, the lead body typically does have an overall insulationcovering. The uninsulated adjacent conductors coils tend to short out inmany places, particularly when the lead is going around sharp, torturousbends in a venous system. In accordance with the present invention andas shown in FIG. 28, there are two inductive coil-parasitic capacitancebandstop filter sections 146 a and 146 b in series with the implantedlead conductors 104 and 104′ that is coiled, wherein the inductiveconductors do have a insulative coating with a specific dielectricconstant material coated over them. These inductive coils L aregenerally closely spaced (with a predetermined spacing) such that adistributed capacitance is formed between the adjacent coils/windings ofthe inductor. Along the length of both bandstop filters 146 a and 146 ban inductance (inductor) is formed along with the parallel parasitic (orstray) capacitance from turn to turn in order to form the parallelresonant inductive-capacitive bandstop (tank) filters of the presentinvention.

As used herein, the terms “parasitic capacitance” and/or “straycapacitance” are synonymous and refer to the capacitance formed betweenthe adjacent turns of an inductive coil L. In addition, as used herein,the terms “parasitic capacitance” and/or “stray capacitance” can alsorefer to the total capacitance formed in the inductor coil L which isthe sum of all of the individual turn to turn capacitances.Electrically, in FIG. 28, the total capacitance Cp=Cp1+Cp2+ . . . Cpnappears in parallel with the total inductance L of the inductive coil toform the parallel resonant L-C bandstop filters 146 and 146′ of FIG. 27of the present invention.

In a preferred embodiment, these inductive coil-parasitic capacitanceself-resonant bandstop filters 146 and 146′ are located at, near orwithin the distal electrodes to the implantable lead. In the case ofFIG. 28, these are the bipolar Tip and/or Ring electrodes 142 and 144 ofa typical cardiac pacemaker. Each bandstop filter 146 a and 146 b is asingle inductive coil component with enough parasitic capacitances to beself-resonant at the MRI RF pulsed frequency or frequency range.Accordingly, FIG. 27 is also the equivalent circuit schematic of FIG.28. The capacitance C, as illustrated in FIG. 27, is the sum Cp of theparasitic capacitances CP1 through Cpn. In this case, “n” indicates thatany number of turns, as desired, can be created for the inductive coilportion of the leads of 104 and 104′. The self-resonant inductor(bandstop filter 146 a and 146 b) portions of the implanted leads 104and 104′ can be formed at the same time the overall lead conductor isformed, or they may be prefabricated (coiled) and then installed in oneor more locations along the lead by laser welding attachment or thelike. In an embodiment, the dielectric insulation that coats the coilsof the bandstop filter portions may also coat the entire lead conductorcoils (this would facilitate easy fabrication in some cases).

Accordingly, it will be appreciated that the inductance may beinherently derived from the lead's material of construction orstructure. Similarly, the capacitance may also be inherently derivedfrom the lead's material of construction or structure.

From the foregoing it will be appreciated that the present inventionrelates broadly to implantable leads which include at least one bandstopfilter comprising at least a portion of the lead, for attenuating RFcurrent flow through the lead at a selected center frequency or across arange of frequencies. The bandstop filter comprises a capacitance inparallel with an inductance, wherein values of capacitance andinductance are selected such that the bandstop filter attenuates RFcurrent flow at the selected center frequency or across the range offrequencies.

Although several embodiments of the invention have been described indetail, for purposes of illustration, various modifications of each maybe made without departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited, except as by theappended claims.

What is claimed is:
 1. An implantable medical lead comprising: at leastone bandstop filter comprising at least a portion of the lead, whereinthe bandstop filter comprises a capacitance in parallel with aninductance, wherein values of capacitance and inductance are selectedsuch that the bandstop filter attenuates RF current flows at a selectedcenter frequency or across a range of frequencies; wherein theinductance is inherently derived from the lead's material ofconstruction or structure; and wherein the capacitance is inherentlyderived from the lead's material of construction or structure.
 2. Thelead of claim 1, wherein the capacitance comprises parasiticcapacitance.
 3. The lead of claim 1, wherein the bandstop filtercomprises at least a part of a coiled or spiral inductor portion of thelead, and the capacitance comprises parasitic capacitance betweenadjacent turns of said inductor portion.
 4. The lead of claim 1, whereinthe overall Q of the bandstop filter is selected to balance impedance atthe selected frequency versus frequency band width characteristics. 5.The lead of claim 1, wherein a Q_(i) of the inductance is relativelyhigh and a Q_(c) of the capacitance is relatively low to reduce theoverall Q of the bandstop filter.
 6. The lead of claim 5, wherein theQ_(i) of the inductance is relatively high by lowering resistive loss inthe inductance.
 7. The lead of claim 5, wherein the Q_(c) of thecapacitance is relatively lowered by raising equivalent seriesresistance of the capacitance.
 8. The lead of claim 1, wherein a Q_(i)of the inductance element a Q_(c) of the capacitance element areselected such that the overall Q of the bandstop filter attenuatescurrent flow through the lead at the selected center frequency or acrossthe range of frequencies.
 9. The lead of claim 8, wherein the range offrequencies includes a plurality of MRI RF pulsed frequencies.
 10. Thelead of claim 1, wherein the bandstop filter is disposed at, within oradjacent to a distal Tip electrode.
 11. The lead of claim 10, whereinthe bandstop filter is integrated into a selected one of a Tip electrodeor a Ring electrode.
 12. The lead of claim 11, wherein the overall Q ofthe bandstop filter is lowered to attenuate current flow through thelead across the range of frequencies.
 13. The lead of claim 1, wherein aQ_(i) of the inductance is relatively low and a Q_(c) of the capacitanceis relatively high to reduce the overall Q of the bandstop filter.
 14. Amedical lead, comprising: a conductor and an electrode contactable withbiological cells; and at least one bandstop filter comprising at least aportion of the lead conductor, for attenuation RF current flow throughthe lead conductor, the bandstop filter further comprising a capacitancein parallel with an inductance; wherein the bandstop filter comprises atleast a part of a coiled or spiral inductor portion of the leadconductor, and the capacitance comprises parasitic capacitance betweenadjacent turns of said inductor portion; and wherein values ofcapacitance and inductance have been selected such that the bandstopfilter attenuates RF current flow at a selected center MRI RF pulsedfrequency or across a range of frequencies.
 15. The device of claim 14,wherein the overall Q of the bandstop filter is selected to balanceimpedance at the selected frequency versus frequency band widthcharacteristics.
 16. The device of claim 14, wherein the bandstop filteris integrated into a selected one of a Tip electrode or a Ringelectrode.
 17. The device of claim 14, wherein a Q_(i) of the inductanceis relatively high and a Q_(c) of the capacitance is relatively low toselect the overall Q of the bandstop filter.
 18. The device of claim 17,wherein the inductance has a relatively low resistive loss, and whereinthe capacitance has a relatively high equivalent series resistance. 19.The device of claim 18, wherein the Q_(i) of the inductance isrelatively high by lowering resistive loss in the inductance.
 20. Thedevice of claim 18, wherein the Q_(c) of the capacitance is relativelylowered by raising equivalent series resistance of the capacitance. 21.The device of claim 14, wherein a Q_(i) of the inductance element and aQ_(c) of the capacitance element are selected such that the overall Q ofthe bandstop filter attenuates current flow through the lead at theselected center frequency or across the range of frequencies.
 22. Thedevice of claim 14, wherein the overall Q of the bandstop filter islowered to attenuate current flow through the lead across a range ofselected frequencies.
 23. The device of claim 14, wherein the AIMD acomprises cochlear implant, a piezoelectric sound bridge transducer, aneurostimulator, a brain stimulator, a cardiac pacemaker, a ventricularassist device, an artificial heart, a drug pump, a bone growthstimulator, a bone fusion stimulator, a urinary incontinence device, apain relief spinal cord stimulator, an anti-tremor stimulator, a gastricstimulator, an implantable cardioverter defibrillator, a pH probe, acongestive heart failure device, a neuromodulator, or a cardiovascularstent.